Ferromagnetic Superconductor UGe2 Research Articles

Ferromagnetism in structurally disordered UFe0.39Ge2

A polycrystalline sample of the uranium ternary germanide UFe0.39Ge2 was examined by means of X-ray powder diffraction, DC magnetization and AC magnetic susceptibility, heat capacity and electrical resistivity measurements performed in wide ranges of temperature and magnetic fields. The experiments confirmed that the compound crystallizes with orthorhombic crystal structure of the CeNiSi2-type that is closely related to that of the ferromagnetic superconductor UGe2 and orders ferromagnetically at 37 K. Moreover, it exhibits some features characteristic of ferromagnetic cluster glasses. The electrical transport in UFe0.39Ge2 is strongly influenced by structural disorder arising from the partly occupied Fe-sites in its crystallographic unit cell.

Anisotropic Magnetic Fluctuations in Ferromagnetic Superconductor UGe2: 73Ge-NQR Study at Ambient Pressure

We report a 73Ge nuclear quadrupole resonance reinvestigation of the ferromagnetic (FM) superconductor UGe2 at ambient pressure. Careful measurement and analysis of the paramagnetic (PM) spectrum taking into account the calculated electric field gradient (EFG) resulted in modification of the previously reported site assignments for three inequivalent Ge sites. The crossover anomaly at Tx in the FM state is not clearly seen in the temperature dependence of the static internal field at the Ge site, whereas it appears in the nuclear spin–lattice relaxation rate, 1/T1. A remarkable site dependence of 1/T1 is observed among the inequivalent Ge sites, where the directions of the principal axes of the EFG are different. This result reveals that the magnetic fluctuation of UGe2 is highly anisotropic, which is consistent with the presence of Ising magnetic fluctuations in the PM state.

Strong coupling between localized 5f moments and itinerant quasiparticles in the ferromagnetic superconductor UGe2**Project supported by the National Natural Science Foundation of China (Grant No. 11404297), the Science Challenge Project (Grant No. TZ2016004), and the Science and Technology Foundation of China Academy of Engineering Physics (Grant Nos. 2013B0301050 and 2014A0301013).

The heavy fermion physics arises from the complex interplay of nearly localized 4f/5f electrons and itinerant band-like ones, yielding heavy quasiparticles with an effective mass about 100 times (or more) of the bare electrons. Recently, experimental and theoretical investigations point out a localized and delocalized dual nature in actinide compounds, where itinerant quasiparticles account for the unconventional superconductivity in the vicinity of a magnetic instability. Here we report the strong coupling between localized 5f moments and itinerant quasiparticles in the ferromagnetic superconductor UGe2. The coupling is nearly antiferromagnetic. As embedded in the ferromagnetic matrix of localized 5f moments below , this coupling leads to short-range dynamic correlations of heavy quasiparticles, characterized by fluctuations of magnetic clusters. Those cluster-like spins of itinerant quasiparticles show a broad hump of magnetization at , which is typical for the spin-glass freezing. Thus, our results present the direct observation of itinerant quasiparticles coexisting with localized 5f moments by conventional magnetic measurements, providing a new route into the coexistence between ferromagnetism and superconductivity in heavy fermion systems.

73Ge-NMR study on magnetic fluctuations of ferromagnetic superconductor UGe2

We report 73Ge-NMR measurement on the ferromagnetic superconductor UGe2 at ambient pressure. The observed NMR spectrum supports that the electric field gradient at three inequivalent Ge sites is correctly deduced by a LDA calculation. The temperature dependences of the nuclear spin lattice relaxation rate 1/T1 for H0⊥a (easy axis) and H0∥a were obtained for the oriented sample. The contrasting behavior in 1/T1 for H0⊥a and H0∥a reveals that the magnetic fluctuation of UGe2 is highly anisotropic.

Piston cylinder cell for high pressure ultrasonic pulse echo measurements.

Ultrasonic techniques such as pulse echo, vibrating reed, or resonant ultrasound spectroscopy are powerful probes not only for studying elasticity but also for investigating electronic and magnetic properties. Here, we report on the design of a high pressure ultrasonic pulse echo apparatus, based on a piston cylinder cell, with a simplified electronic setup that operates with a single coaxial cable and requires sample lengths of mm only. The design allows simultaneous measurements of ultrasonic velocities and attenuation coefficients up to a pressure of 1.5 GPa. We illustrate the performance of the cell by probing the phase diagram of a single crystal of the ferromagnetic superconductor UGe2.

High pressure XANES and XMCD in the tender X-ray energy range

ABSTRACTWe have developed an experimental setup at the ESRF beamline ID12 dedicated to X-ray absorption and magnetic circular dichroism measurements at high pressure adapted for the tender X-ray energy range and compatible with low temperatures and with high magnetic field. The focused incoming X-ray beam passes through a thin diamond disk attached to a fully perforated diamond anvil and X-ray fluorescence photons from the sample are collected in back-scattering geometry through the same diamond disk. The pressure in the cell is measured using the ruby luminescence through a full diamond anvil. The highest pressure attainable with this diamond anvil cell (DAC) depends on the thickness of the diamond disk and it is above 16 GPa for a 80-μm thick plate and exceeds 4.5 GPa in the case of 30-μm diamond disk. Excellent performances of this setup in the tender X-ray range are illustrated with X-ray absorption near-edge structure studies of the phase transitions in KCl at the potassium and chlorine K-edges (3.61 and 2.82 keV, respectively) as well as in CdS at the sulfur K-edge (2.47 keV). This DAC together with a dedicated total fluorescence yield (TFY) detector could be mounted in the main heat exchanger of a cryostat and inserted in a bore of a superconducting magnet, this makes possible to perform X-ray magnetic circular dichroism (XMCD) experiments at low temperature, high magnetic field and high pressure. Feasibility of this approach is shown with the XMCD results obtained at the U M-edges in ferromagnetic superconductor UGe.

Magnetic excitations in the ferromagnetic superconductor UGe2 under pressure

Ferromagnetism and superconductivity coexist in UGe2 under pressures in the range of 10-16 kbar. Here, equal spin electrons are paired to give a spin triplet state but the pairing mechanism is still not verified experimentally. The work presented here attempts to verify whether the longitudinal magnetic fluctuations associated with a magnetic transition within the ferromagnetic state might be responsible for the electron pairing. We show that the energy scale of these fluctuations must be smaller than those associated with the second order phase transition at the Curie temperature. Furthermore, there is no significant change of the energy scale of the fluctuations at the Curie temperature as a function of pressure.

Fermi surface, magnetic, and superconducting properties in actinide compounds

Abstract The de Haas–van Alphen effect, which is a powerful method to explore Fermi surface properties, has been observed in cerium, uranium, and nowadays even in neptunium and plutonium compounds. Here, we present the results of several studies concerning the Fermi surface properties of the heavy fermion superconductors UPt3 and NpPd5Al2, and of the ferromagnetic pressure-induced superconductor UGe2, together with those of some related compounds for which fascinating anisotropic superconductivity, magnetism, and heavy fermion behavior has been observed.

Ferromagnetic Quantum Critical Endpoint in UCoAl

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at H m ∼0.7 T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature T 0 ∼11 K. With increasing pressure, H m linearly increases, while T 0 decreases and is suppressed at the quantum critical endpoint (QCEP, P QCEP ∼1.5 GPa, H m ∼7 T). At higher pressure, the value of H m identified as a crossover continuously increases, while a new anomaly appears above P QCEP at higher field H * in resistivity measurements. The field dependence of the effective mass ( m * ) obtained by resistivity and specific heat measurements exhibits a step-like drop at H m at ambient pressure. With increasing pressure, it gradually changes into a peak structure and a sharp enhancement of m * is observed near the QCEP. Above P QCEP , the enhancement of m * is reduced, and a broad plateau is found between H m and H * . We compare our results on UCoAl with those of the ferromagnetic superconductor UGe 2 and the itinerant metamagnetic ruthenate Sr 3 Ru 2 O 7 .

Neutron scattering studies of the lattice expansion in a ferromagnetic superconductor UGe2 under pressure

We report high-resolution measurements of the lattice constants of UGe2 under pressure probed by a novel technique which utilises Larmor precession of polarised neutrons and surpasses the resolution of conventional scattering methods by two orders of magnitude. We confirm the presence of sharp anomalies in the lattice parameters at both the Curie and crossover temperatures at ambient pressure. We find that for pressure of 9.3 kbar the anomaly at the Curie temperature shifts to lower temperature in agreement with the known phase diagram. At 9.3 kbar, the pressure corresponding to an onset of superconductivity, the lattice expansion at the ferromagnetic transition is much stronger than at ambient pressure. The results indicate a complex evolution of the electronic structure of UGe2 with pressure and suggest that magneto-elastic coupling is strengthened at the pressures at which superconductivity appears; magneto-elastic coupling therefore may play an important role in stabilising superconductivity.

Magnetic field evolution of critical end point in UGe2

New measurements on the ferromagnetic heavy-fermion superconductor UGe2 are reported. Thermal expansion measurements via strain gage allow us to locate precisely the critical end point (CEP) of the first order boundary between the large moment ferromagnetic phase FM2 (M0 = 1.5 μB) and the small moment phase FM1 (M0 = 0.9 μB). The field dependence of the CEP is compared with that of the paramagnetic-FM1 transition above the tricritical point.

The pairing symmetry in the ferromagnetic superconductor UGe2

AbstractWe give an extensive treatment of the pairing symmetry in the ferromagnetic (FM) superconductor UGe2. We show that one can draw important conclusions concerning the superconducting state, considering only the transformation properties of the pairing function, without assumptions about the form of the pairing amplitudes.

Positive muon spin rotation and relaxation measurements on the ferromagnetic superconductorUGe2at ambient and high pressure

Results of a detailed investigation of the ferromagnetic superconductor UGe2 using positive muon spin rotation and relaxation techniques are presented. The pressure and temperature dependences of the frequencies and related spin-spin relaxation rates show that the transition from the weakly to the strongly polarized magnetic (WP-SP) phases is still observable at TX?3?K under a pressure of 1.33(2) GPa. Thus this transition survives at higher pressures than previously believed. The temperature TX at 1.00(2) GPa corresponds to a thermodynamic phase transition rather than a crossover. No such statement can be given reliably at lower pressure. A substantial shrinking of the component along the easy axis of the diagonal hyperfine tensor, at the muon site where it is large, is observed in the SP phase relative to the WP phase. This corresponds to an appreciable decrease in the electronic density at the Fermi level in the SP phase. The investigation of the paramagnetic-ferromagnetic critical spin dynamics at ambient pressure and at 0.95(2) GPa shows that the simple one-band model is an oversimplification inconsistent with our critical spin-dynamics results. Data from specific heat, Fermi-surface studies, Hall effect, neutron form factor, and spectroscopic techniques supports this conclusion. Even at 0.95(2) GPa the conduction electrons are characterized by a small magnetic moment, relative to the bulk magnetization per uranium atom.

Mass enhancement and reentrant ground state under magnetic field in heavy fermion superconductors

Three different cases of magnetic field induced reentrant ground states in heavy fermion systems URhGe, UGe2 and CeRhIn5, recently studied in Grenoble, are discussed. URhGe is a ferromagnetic superconductor with the reentrace of superconductivity under magnetic field (H) which is associated to a spin reorientation. In the ferromagnetic superconductor UGe2 an enhancement of the superconducting upper critical field Hc2 appears at its metamagnetic transition between two ferromagnetic phases. CeRhIn5 is a superconductor under high pressure and reentrance of antiferromagnetism is induced by magnetic field. We analyze the links between the H enhancement of the different contributions to the effective mass and the field reentrant phase.

Interplay of Superconductivity and Ferromagnetism in UGe2

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe2 is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (TC,TFM), where TC and TFM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe2 is quite satisfactory.

Reinvestigation of superconducting phase diagram of [formula omitted] by AC magnetic susceptibility experiment

We report a superconducting phase diagram of the ferromagnetic superconductor UGe 2 investigated by AC magnetic susceptibility measurements. In contrast to previous phase diagrams, we found that the superconducting transition temperature and volume fraction show a “M-shaped” structure as a function of pressure. From this observation, we suggest that both of two critical points will play a crucial role in the occurrence of superconductivity in UGe 2 .

Phonon spectrum of the ferromagnetic superconductorUGe2and consequences for its specific heat

We report inelastic neutron scattering measurements of the phonon spectrum of the pressure-induced ferromagnetic superconductor UGe$_{2}$. No changes of the spectrum were found on cooling down to low temperature. The phonon contribution to the specific heat was estimated from a fit to our data. The excess specific heat previously noted at around $T_{x} \approx$ 30 K is not due to phonons but is well described by the temperature dependence of the magnetic order parameter at the molecular field level.

Unconventional superconductivity in the itinerant ferromagnet UGe 2: 73Ge-NQR study under pressure

The superconducting characteristics on the itinerant ferromagnetic superconductor UGe 2 have been investigated via the 73Ge-NQR measurements under pressure ( P) near the critical pressure P x where the first-order transition takes place from the low-temperature ( T) and low- P ferromagnetic phase (FM2) to high- T and high- P one (FM1). The measurements of nuclear spin-lattice relaxation rate 1 / T 1 have revealed a power-law like behavior as T n without the coherence peak, whereas at low temperatures, it deviates from the T n dependence to a T-linear like behavior. These results that depend on the pressure are well understood in terms of non-unitary spin-triplet pairing state where the ferromagnetic up-spin band is gapped, but the down-spin band remains gapless at the Fermi level. We will argue an intimate relationship of the onset of SC with FM1 and FM2.

Soft X-ray Absorption Magnetic Circular Dichroism Study of Ferromagnetic Superconductor UGe2

X-ray absorption magnetic circular dichroism (XMCD) at the U N 4,5 edges was measured for the ferromagnetic superconductor UGe 2 in the normal state. The orbital and spin magneic moments deduced from the sum rule analysis of the XMCD data indicate that the U atom in UGe 2 is considered to be closer to the trivalent state rather than the tetravalent state. The temperature dependence of the XMCD signal does not show any significant anomaly at T * ∼30 K, where the resistivity anomaly was observed at ambient pressure. The XMCD measurement at the U N 2,3 edge indicates that the U 6 d electrons have negligibly small magnetic contributions.

Single Crystal Synchrotron X-Ray Diffraction Study under Pressure in UGe2

Synchrotron radiation x-ray diffraction experiments on the pressure-induced ferromagnetic superconductor UGe 2 has been performed using a diamond anvil cell (DAC) diffractometer to study the correlation between the crystal structure and ferromagnetism. The oscillation photos covering the three-dimensional reciprocal spaces were taken both below and above a critical temperature T x at pressures up to 1.1 GPa. Coupled charge density wave (CDW) and spin density wave (SDW) fluctuations were predicted as an origin of unusual properties of UGe 2 by Watanabe and Miyake, but no additional superlattice Bragg peaks corresponding to the CDW reflections could be observed, resulting in an upper limit of its intensity of ∼1 ×10 -4 to the fundamental Bragg peak intensity.